Sen. Sherrod Brown (D-Ohio) holds a substantial lead over Republican challenger Josh Mandel with six weeks left until Election Day, according to a new Washington Post poll released Tuesday, giving Democrats some breathing room in a race where outside groups have put nearly $20 million toward defeating the incumbent.

Brown leads Mandel 53 percent to 41 percent among likely Ohio voters. Among registered Ohio voters, Mandel -- the state treasurer -- trails Brown 51 percent to 39 percent.

Brown’s level of support is nearly identical to President Obama’s. But Mandel’s is a tick lower than Mitt Romney’s. Among likely Ohio voters, Obama leads Mitt Romney 52 percent to 44 percent. Among registered voters, Obama enjoys a slightly wider 52 percent to 41 percent lead over Romney. The poll shows voters are less certain about their Senate choice than they are about their presidential preference.

Mandel runs behind Romney in a few key respects. Mandel earns the support of 85 percent of Republicans, compared to 91 percent for Romney. He is viewed favorably by 76 percent of all Republican voters, compared to 90 percent who hold a favorable opinion of the GOP presidential nominee. Mandel claims the support of 54 percent of  white evangelical Christians who are likely to vote – a subset of the electorate that generally favors Republicans – a shade lower than 63 percent who support Romney.

Mandel, an Iraq War veteran, was regarded as a top Republican recruit last year, as he moved toward officially declaring his bid against Brown. He caught the attention of national strategists after he was elected treasurer by a wide margin in 2010. The youthful looking 34-year-old has been a prolific fundraiser; even outpacing Brown ‘s quarterly hauls at times.

But the Republican has also absorbed some blows to his image. A March trip to the Bahamas for a fundraiser and speech to payday lending industry leaders earned him some unflattering headlines and became fodder for Democratic attacks. His decision to hire young and inexperienced staffers from his 2010 campaign for high-ranking positions in the treasurer’s office was another distraction. Democrats have also sought to cast Mandel as an absentee treasurer who hasn’t been attending to his official duties.

Still, Mandel remains standing against Brown, thanks in no small part to Republican outside groups, which have dropped over $19 million on the race. Crossroads GPS, the powerful nonprofit group tied to Republican operative Karl Rove, released the latest ad Tuesday morning. The spot ties Brown to Obama, a tactic Republicans have often used in attack ads against the senator.

While Mandel's standing among Republicans leaves room for improvement, he is keeping pace with Brown among independents. Mandel claims the support of 43 percent of unaffiliated voters to the Democrat’s 46 percent.

Likely voters are split when it comes to their opinion of Mandel, with 42 percent having a favorable opinion of him and 45 percent holding an unfavorable view of the treasurer. Fully 49 percent of likely voters percent say they hold a favorable view of Brown, while 40 percent give unfavorable ratings to him. Both candidates are unknown to a significant portion of Ohio voters, with 11 percent saying they have no opinion of Brown and 13 percent saying they have no opinion of Mandel. By contrast, only 2 percent of Ohio likely voters are unsure about Romney, 1 percent Obama.

Other recent polls have shown Brown leading, including a Columbus Dispatch/Ohio Newspaper organization survey conducted earlier this month. In that survey, Brown led Mandel 52 percent to 45 percent. A recent NBC/Wall Street Journal/Marist poll showed Brown leading Mandel 49 percent to 42 percent.

The Washington Post poll of Ohio was conducted from Sept. 19-23. Among the sample of 934 registered voters, the margin of error is plus or minus 4 percentage points. Among the sample of 759 likely voters, the margin of error is plus or minus 4.5 percentage points. Click here for interactive poll results as well as a exact question wording and poll methodology.